The present invention relates to a process for monitoring binder, particularly isocyanate-based binders, dosage and distribution on a surface by ultraviolet fluorescence and to apparatus useful therefor.
Various types of binders have been used to produce engineered composite materials such as oriented strand board. Suitable binders include phenol formaldehyde resins and isocyanates, particularly polymeric diphenylmethane diisocyanate (“PMDI”). In producing such engineered composites, the binder is generally applied to a material such as wood fibers, wood strands, wood flakes or some other lignocellulose-based material. Ideally, the amount of binder applied (“dosage”) would be sufficient to cover 100% of the surface of 100% of the wood fibers, wood strands, etc. (“distribution”). In most commercial processes, an excess of binder is used to ensure sufficient distribution. Longer than necessary mixing times may also be used to ensure that the binder is sufficiently distributed so that weak spots in the composite material due to insufficient adhesion do not occur. This use of excess binder and extended mixing times significantly increases the cost of producing engineered composite materials.
It would therefore be advantageous to develop a method for determining binder dosage and distribution during the composite production process with sufficient accuracy that use of excess binder and extended mixing times are unnecessary.
Spectroscopic methods for making such determinations have been investigated by those seeking to improve the production of composite materials. Solid NMR characterization of the bonding of composite materials was studied by Frazier and Wendler and the results were presented in “15N CP/MAS NMR analysis of pMDI bonded cellulose composites” presented at the 48th Annual Meeting of the Forest Products Society, Portland, Me., Jun. 26-29, 1994. Sun et al attempted to correlate fluorescence intensity changes with FTIR spectra generated by monitoring the disappearance of the isocyanate group during the reaction which occurs in the commercial production process. (See, e.g., Sun et al, Institute of Materials Science, Storrs Report TR-38-ONR, Connecticut University (1994).)
UV absorption and fluorescence spectroscopy are also techniques which have been evaluated for their usefulness in monitoring urethane-forming reactions. However, until now, methods utilizing such UV spectroscopic techniques were not capable of providing real time, macroscopic imaging of the composite material as it was being produced.
For example, F. Kamke's work reported in “Wood Based Composites Program Annual Report” (Jun. 1, 1994-May 31, 1995) was a microscopic study of UV fluorescence imaging of polymeric MDI resin distribution on wood strands. Kamke states that because polymeric MDI fluorescence is very weak, a very intense UV source (specifically, a 100 watt mercury vapor lamp) and signal averaging of many video frames to reduce noise level were necessary. Although signal averaging to reduce noise level works well for stationary samples, it is not very useful when the material being evaluated is moving on a conveyor belt and the video image is constantly changing. Microscopic evaluation of a material is also impractical for monitoring a commercial production process because of the great potential for variation between samples. The Kamke method would not therefore be practical for monitoring a commercial process for the production of a composite material.
Yu et al report a technique in which naphthylene diisocyanate is used as a molecular sensor to monitor cure reactions in a polyurethane in U.S. Pat. No. 4,885,254. Yu et al correlate the fluorescence intensity and overall extent of reaction between 1,5-naphthyl diisocyanate and n-butanol. This correlation was established by identifying the various species present during the urethane-forming reaction using HPLC that was confirmed by IR spectra. The UV-visible absorption spectrum and fluorescence spectrum for each of these species were then generated. Shifts in the UV-visible spectrum were observed as the naphthyl diisocyanate reacted to form the monourethane and diurethane. The fraction of each species present at a given time was determined by linear regression analysis. The extent of the reaction was calculated from UV spectral analysis. A correlation between the experimentally determined fluorescence intensity at 357 nanometers and the calculated overall extent of reaction derived from UV spectral analysis was made.
U.S. Pat. No. 5,100,802 discloses a method for measuring the rate and extent of cure of a resin system in which a fluorescent dye is added to the system being polymerized.
U.S. Pat. No. 4,922,113 discloses a method for monitoring a coating's weight, uniformity and surface defects in which a UV-escer that absorbs radiant energy is included in the coating composition. The radiant energy emitted by the coating at the same wavelength as energy emitted by the UV-escer can be detected and correlated to pre-established standards.
U.S. Pat. No. 4,651,011 discloses a method for determining the extent of cure of a polymer. In this method, the degree of free space rotation of a fluorospore added to the polymer system is determined by fluorescent measurement of the fluorospore.
To date, however, no method for determining binder dosing, particularly isocyanate-based binder dosing and distribution during actual production of composite materials without adding some type of “marker” such as a dye, fluorospore or UV-escer has been developed.